The present invention relates generally to semiconductor fabrication and, more particularly, to a method for reducing the amount of perfluorocompound gas contained in exhaust emissions from plasma processing, a method for treating perfluorocompound gas contained in exhaust emissions from plasma processing, and a method for forming an integrated circuit.
Despite efforts throughout the world to ban the use of chlorofluorocarbon gases (CFCs), global warming remains a concern. Thus, efforts to reduce the adverse effect of gases other than CFCs have increased in recent years. In the area of semiconductor fabrication, much attention has been focused on the adverse effect exhaust emissions from plasma processes have on global warming. In particular, efforts are being made to reduce the amount of perfluorocompound gases (PFCs) in exhaust emissions from plasma processes, even though the emission levels of PFCs from plasma processing are far lower than emission levels of CO2 from combustion sources. These efforts are being made because even relatively small amounts of PFCs such as CF4 and C2F6 have a significant impact on global warming because of their longevity in the atmosphere. Indeed, some PFCs persist in the atmosphere for thousands of years and, as a consequence of such longevity, the global warming potential (GWP) of, for example, SF6 is about 24,000 times greater than that of CO2. Thus, techniques for reducing the emission of PFCs from plasma processes are currently being studied.
The optimal way to reduce the emission of PFCs from plasma processes would be to replace PFCs with alternative gases. At present, however, the replacement of PFCs with other gases is not technically feasible. Another way to avoid the emission of PFCs from plasma processes would be to capture and recover all emitted PFCs. Unfortunately, however, the capture and recovery of all emitted PFCs is not currently an economically viable alternative for semiconductor manufacturers. Thus, the most attractive technique for reducing the emission of PFCs from plasma processes in the near term is abatement, which involves the conversion of PFCs into less harmful forms before venting the exhaust gases to the atmosphere.
As reported by Marci Liao, Kent Wong, and Jim McVittie in a presentation entitled xe2x80x9cInductive Coupled Plasmas for Point-of-Use PFC Abatement,xe2x80x9d (Plasma Etch Users Group (PEUG) Talk, Feb. 17, 1998), one potential PFC abatement scheme involves plasma oxidation of PFCs in a secondary plasma processing chamber. In this PFC abatement scheme exhaust emissions from a plasma processing chamber are carried by the pumping action of a turbopump into an inductively coupled plasma processing chamber in which a secondary plasma is generated. Oxygen is added to the secondary plasma to oxidize the PFCs in the exhaust emissions into gases having lower GWP. From the inductively coupled plasma processing chamber, the thus-treated exhaust emissions are carried by the pumping action of a backing pump into a scrubber. After passing through the scrubber, the exhaust emissions are vented to the atmosphere.
In a plasma oxidation PFC abatement scheme such as that described above a relatively high flow rate of oxygen is required to abate a selected percentage, e.g., 85%, of the PFCs from the exhaust emissions. The introduction of large amounts of oxygen is disadvantageous because it increases the loading on the vacuum pump in the plasma processing system, which may substantially increase the operating costs for the system. Furthermore, in the event a more powerful vacuum pump is required to handle this increased loading, significant capital expenditures must be incurred to obtain such a pump and retrofit it into the plasma processing system. In light of these significant cost and retrofittability issues, a plasma oxidation PFC abatement scheme may not be a preferred solution for semiconductor manufacturers.
In the PFC abatement scheme described above, hydrogen may be a more effective abatement gas than oxygen. In other words, by using hydrogen as the abatement gas rather than oxygen, the selected percentage, e.g., 85%, of the PFCs from the exhaust emission may be abated using a lower flow rate of the abatement gas. Thus, from the standpoint of minimizing increased loading on the vacuum pump in the plasma processing system, the substitution of hydrogen as the abatement gas appears promising. Unfortunately, however, hydrogen (H2) and methane (CH4), which may be formed when hydrogen reacts with PFCs, are hazardous gases and require the use of costly double-contained piping. As such, the use of hydrogen as an abatement gas for PFCs is undesirable because it is not only expensive, but also raises significant safety issues.
In view of the foregoing, there is a need for a method for reducing the amount of PFCs contained in exhaust emissions from plasma processing that does not require substantially increased vacuum pump capacity in the plasma processing system and does not present significant safety hazards.
Broadly speaking, the present invention fills this need by providing a plasma processing system in which water vapor is used as an abatement gas to reduce the amount of perfluorocompound gas in exhaust emissions from plasma processing.
In accordance with one aspect of the present invention, a method for treating perfluorocompound gas contained in exhaust emissions from plasma processing is provided. In this method a plasma abatement device is first provided downstream of a plasma processing chamber. Next, perfluorocompound gas contained in exhaust emissions from the plasma processing chamber is channeled into the plasma abatement device. A gas containing water vapor is then introduced into the plasma abatement device. By way of example, the perfluorocompound gas contained in the exhaust emissions may be CF4, C2F6, CHF3, SF6, NF3, C3F8, C4F8, and mixtures thereof.
In one embodiment, the plasma abatement device is a secondary plasma processing chamber. In this embodiment, the operation of introducing the gas containing water vapor into the plasma abatement device preferably includes selecting a percentage of perfluorocompound gas to be abated, determining a minimum flow rate of water vapor needed to abate the selected percentage of perfluorocompound gas, and introducing the gas containing water vapor into the plasma abatement device at the determined minimum flow rate. The gas containing water vapor preferably includes at least about 10% of water vapor.
In accordance with another aspect of the present invention, a method for reducing the amount of perfluorocompound gas contained in exhaust emissions from plasma processing is provided. In this method exhaust emissions from plasma processing containing a perfluorocompound gas are contacted with a gas containing water vapor.
In one embodiment, the exhaust emissions from plasma processing containing the perfluorocompound gas are contacted with the gas containing water vapor in a plasma abatement device provided downstream of a plasma processing chamber. In this embodiment, the plasma abatement device is preferably a secondary plasma processing chamber. In another embodiment, the exhaust emissions from plasma processing containing the perfluorocompound gas are contacted with the gas containing water vapor directly in the plasma processing chamber. This embodiment may be used only where the water vapor does not significantly degrade the performance of a plasma process in the plasma processing chamber. In either of these embodiments, the operation of introducing the gas containing water vapor is conducted at the determined minimum flow rate needed to achieve the selected percentage of abatement, as set forth above.
In accordance with yet another aspect of the present invention, a method for forming an integrated circuit is provided. In this method a semiconductor wafer is first provided. Next, the semiconductor wafer is subjected to plasma processing in a plasma processing chamber. The exhaust emissions from plasma processing containing a perfluorocompound gas are then contacted with a gas containing water vapor to reduce the amount of perfluorocompound gas contained in the exhaust emissions. The exhaust emissions may be contacted with the gas containing water vapor in either a plasma abatement device or directly in the plasma processing chamber, as set forth above.
The present invention provides a method that uses water vapor to reduce the amount of perfluorocompound gas contained in exhaust emissions from plasma processing. This method is advantageous because water vapor, which is a benign gas, dissociates into hydrogen and oxygen in a plasma. As such, the present invention achieves the benefits associated with the addition of hydrogen, e.g., efficient abatement of perfluorocompound gas, without the associated safety hazards. Furthermore, because water vapor abates perfluorocompound gas more efficiently than oxygen, the method of the invention can achieve an acceptable percentage of abatement at a flow rate that does not exceed the capacity of the pump typically used in commercial plasma processing systems. Thus, the method is further advantageous because it enables the abatement of perfluorocompound gas to be achieved without significantly increasing the operating costs of the plasma processing system or requiring significant capital expenditures to be incurred to obtain a more powerful pump and retrofit such pump into the system.
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.